1. Field of the Invention
The present invention relates to a semiconductor storage device having an electrically rewritable, nonvolatile memory, and to a method of fabricating the semiconductor storage device.
2. Description of the Related Art
In a conventional semiconductor storage device, a selection transistor is set in series with a floating gate transistor, and the floating gate electrode of this floating gate transistor is capacitatively coupled via a thin tunnel oxide film on a high-concentration N-type diffusion region formed on a bulk substrate, and a single poly flash EEPROM (Electrically Erasable and Programmable Read Only Memory) is formed. Voltage is applied to the control line of the floating gate transistor, the source layer thereof is grounded, and the data of the storage elements is erased (refer to, for example, Japanese National Publication No. 2003-501806, and mainly, page 8, paragraphs 0015-0016, and FIG. 2 and FIG. 4).
Further, there is also a technique in which a flash memory cell is structured by connecting, in series at one floating gate electrode, two transistors which are formed respectively at a P well layer and an N well layer forming a bulk substrate. In the erasing operation, the control gate electrode and drain layer are grounded, voltage of 7 V is applied to the source layer, electrons are removed from the floating gate electrode in the tunnel current, and the threshold voltage of the flash memory is lowered. In the writing operation, the drain layer is grounded, voltage of 5 V is applied to the control gate and the source layer, hot electrons are injected in the floating gate electrode, the threshold voltage of the flash memory is raised, and the stored data is read-out in accordance with the magnitude of the threshold voltage (refer to, for example, Japanese Patent Application Laid-Open (JP-A) No. 2001-229690, page 5, paragraph 0011 to page 6, paragraph 0021, and FIG. 1).
In both of the above-described conventional techniques, a single poly flash EEPROM or a flash memory is formed on a bulk substrate. Therefore, the withstand voltage between the source and drain is high, and a writing operation such as described above is possible. However, in a semiconductor storage device of a fully depleted type SOI structure in which an SOI (Silicon On Insulator) layer formed from a silicon thin film is layered on a buried oxide film, there are the problems that it is difficult to sufficiently ensure the withstand voltage between the source and drain of a MOSFET (MOS Field Effect Transistor) formed at the SOI layer, charges cannot be injected through a gate oxide film into the floating gate electrode, and, when forming an electrically rewritable, nonvolatile memory at a semiconductor storage device of an SOI structure, writing operation cannot be carried out.